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Second International Symposium on Ultra High Performance Concrete
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Structural performance of precast/prestressed bridge girders built with
ultra high performance concrete
St ruc t ura l Pe rfor m a nc e of
Pre c a st Pre st re sse d Bridge
Girde rs Built w it h U lt ra H igh
Pe rfor m a nc e Conc re t e
N R C C - 5 0 0 6 0
A l m a n s o u r , H . ; L o u n i s , Z ,
2 0 0 8 - 0 3 - 0 7
A version of this document is published in / Une version de ce document se trouve dans:
Second International Symposium on Ultra High Performance Concrete (Kassel, Germany, March 05, 2008), pp. 822-830
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Structural Performance of Precast Prestressed Bridge Girders Built with Ultra High Performance Concrete
Husham Almansour
Research Associate
Institute for Research in Construction National Research Council Canada Ottawa, Ontario, Canada
Zoubir Lounis
Senior Research Officer & Group Leader Institute for Research in Construction National Research Council Canada Ottawa, Ontario, Canada
Structural Performance of Precast Prestressed Bridge
Girders Built with Ultra High Performance Concrete
Summary
With its very high strength, high modulus and very low permeability, ultra high performance concrete (UHPC) can provide major improvements over conventional high performance concrete (HPC) in terms of structural efficiency, durability and cost-effectiveness if used in the construction of slab-on-precast prestressed girder bridges. In this study, the optimum use of UHPC is evaluated for the case of simply supported cast in place concrete slab on precast/prestressed UHPC girder bridges. These bridges are designed according to the Canadian Highway Bridge Design Code requirements, including no cracking at the serviceability limit state. The design of superstructure is achieved through an iterative procedure. The stress distributions and stress concentration zones in the girders under dead loads and traffic loads are investigated. The required number of girders, girder size, and area of prestressing steel are optimized to achieve minimum weight of superstructure. The results show that the use of UHPC enables a significant reduction in the volume of concrete girders from 49% to 65% and a significant reduction in the overall superstructure weight by 32% when compared to HPC bridges.
Keywords: Ultra-high performance concrete, precast/prestressed bridge girder, finite element
modeling, limit states design
1 Introduction
In the last four decades, there was a considerable growth in the use of high strength/high performance concrete (HSC/HPC) in highway bridges. Simply supported slab-on-precast bridge girders is one the most common forms of structural systems used for the construction of highway bridges in North America. However, the benefits of using HSC/HSC to extend the span length or reduce the weight of simply supported precast girder bridge systems reach a limit at about 50 MPa, beyond which there is only marginal improvement as the governing design criterion is the condition of no cracking at service, (i.e. fully prestressed girders) [1,2]. Ultra high performance concrete (UHPC) represents a major development step over HPC, through the achievement of very high strength and very low permeability. The compressive strength of UHPC varies from 120 to 400 MPa, its tensile strength varies from 10 to 30 MPa, and the modulus of elasticity is in the range of 60 – 100 GPa [3,4].
evaluation or design methodology for this type of construction has been developed yet. The first UHPC highway bridge [5] was designed and constructed in France and opened to traffic in 2001 with two simply supported spans of 22 m each. At the same time, another UHPC bridge [6] was constructed in Italy with a span of 11.8 m. More recently, a 33.8 m span UHPC bridge was designed and constructed in Iowa and opened to traffic in late 2005 [7]. The only available design guidelines for UHPC structures are the French recommendations, AFGC-IR-02 [8]. These recommendations provide modifications to the existing French design standards for reinforced and prestressed concrete structures. However, it does not provide detailed design recommendations for highway bridge structures. Hence, there is an urgent need to develop a procedure for the design of UHPC bridges according to the Canadian Highway Bridge Design Code (CHBDC-06) [10] and using the available standard Canadian Prestressed Concrete Institute (CPCI) [9] precast/prestressed I-girder sections. The objective of this paper is to evaluate the structural efficiency of UHPC when used in precast/prestressed girder bridges in terms of reducing the number of required girders and/or size of girders when compared to conventional HPC precast bridge girders.
2 Design of Slab-on- UHPC Girder Bridge Superstructure
In order to illustrate the benefits and efficiency of UHPC bridge girders, a comparative study of the design UHPC and HPC girder bridges is undertaken. Two simply supported bridge superstructures are considered in this study, namely: (i) a typical cast in place concrete slab on precast/prestressed (PC) HPC girders bridge; and (ii) a typical cast in place concrete slab on PC UHPC girders. The total width of the bridge including the barrier walls is 12.45 m and its span length is 45 m. The slab thickness for both bridges is 175 mm, which corresponds to the minimum slab thickness allowed in the Canadian Highway Bridge Design Code.
The traffic load and bridge design comply with all Serviceability and Ultimate Limit States (SLS and ULS) requirements of CHBDC-06. Two types of live loads are applied on the deck surface, the lane loading and a single moving truck. The direction of the movement is reversed between different design lanes. For multi-lane loading, modification factors of 0.9 and 0.8 are applied for two lanes and three lanes, respectively. For the Ultimate Limit States (ULS), the magnification factors for the dead and traffic loads are 1.2 and 1.7, respectively. The material reduction factors are 0.75 and 0.95 for precast concrete and prestressing steel, respectively. At each section, it is ensured that the factored moment and shear force are less or equal to the factored flexural resistance and shear resistance, respectively.
2.1 Design requirements and procedure
In this study, the bridge is designed in accordance with CHBDC-06 regarding the live load model and load factors, however, the resistance factors for UHPC at ULS are conservatively adjusted by referring the AFGC-IR-02 recommendations. The iterative design procedure for
Structural Performance of Precast Prestressed Bridge Girders Built with Ultra High Performance Concrete
feasible superstructure design is determined, a refined analysis is performed using a linear elastic finite element model to check its adequacy. At this stage, the detailed stress distribution is examined to identify the zones of maximum stresses to optimize the girder section and prestressing steel area and layout.
Initial UHPC / HPC Bridge Superstructure Section
Simplified Analysis and Design (CAN/CSA-S6-06 & AFGC-IR-02)
Initial Design Adequacy
Check
Refined Analysis using Finite Element Model
Design Check Change Prestressing
Steel Area and/or Girder size and/or
No of Girders Yes No Yes No Final Bridge Design Change Prestressing
Steel Area and/or Girder Size
Figure 1: Design Procedure for UHPC and HPC Bridges
2.2 Prestressing system for HPC and UHPC girders
The selected prestressing are low-relaxation strands, size 13, Grade 1860, with nominal diameter of 12.7 mm and nominal area of 98.7 mm2 and tensile strength (fpu) of 1860 MPa.
CHBDC-06 limits the minimum effective stress in tendons to 0.45 fpu, the maximum stress at
jacking is limited to 0.78 fpu; the maximum tensile stress at transfer to 0.74 fpu; and the
maximum stress at ultimate to 0.95 fpu The total prestress losses are estimated to be 16.9%
of the tendon strength. The tendons for the HPC and UHPC girders are arranged in straight and conventional deflected strand pattern groups. The straight tendons provide 50% to 60% of the total prestressing steel area, depending on the maximum stresses in the girder. There was no need to debond the strands near supports as the tensile stresses remained below the allowable value.
corresponding girder spacing is 2.5 m and the length of the cantilever slab is 1.225 m on each side. The HPC used for the girders has a compressive strength of 40 MPa; initial compressive strength of 30 MPa, and a modulus of elasticity of 29.3 GPa. The slab is made of normal concrete with of 30 MPa and a modulus of elasticity of 25.6 GPa. The cracking strength of HPC is ) (fc' ) (fci' ) (fc' ' 4 .
0 fc . At transfer and during construction, the allowable
compression stress is and the limit for tensile stress is , where is equal to ' ci f 6 . 0 0.5fcri fcri ' 4 .
0 fci [10]. In the present study, the bridge is designed for no cracking at SLS. The deflection of the bridge for superstructure vibration control is checked in accordance to Cl. 3.4.4 of CHBDC-06 [10]. The main properties of all investigated CPCI sections are summarized in Table 1.
Table 1 Properties of Investigated Standard CPCI I-Sections [9] CPCI Girder A (m2) I (m4) St (m3) Sb (m3)
900 0.218 0.0193 0.0384 0.0486
1200 0.320 0.0539 0.0800 0.1023
1600 0.499 0.1747 0.2166 0.2202
A: cross sectional area, I : moment of inertia, St and Sb: section
modulus with regard to top and bottom fibers, respectively
2.4 UHPC Bridge girder design
It is found that only four girders are needed for the UHPC bridge design (Fig. 2-b). The two smallest CPCI girders are the CPCI 900 and CPCI 1200, which are selected in order to investigate their structural efficiency for use with UHPC. Accordingly, the girder spacing is 3.3 m and the side cantilever slabs of the deck are 1.275 m each. The UHPC used has a compressive strength (f’c) of 175 MPa and a modulus of elasticity of 64.0 GPa. The slab is
made of normal concrete with a compressive strength of 30 MPa and a modulus of elasticity of 25.6 GPa. The allowable tensile strength of UHPC at (SLS) is taken conservatively as
' c
t 0.4 f
f = [13].
At transfer and during construction, the compressive strength is taken as . The allowable compressive stress is . The limit for tensile stress is , where is taken conservatively as MPa fci' =105 cri f 6 . 0 ' 6 . 0 fci fcri ' ci f cri =0.4
f . In the present study, the bridge is designed for no cracking at SLS. The deflection of the bridge for superstructure vibration is also checked in accordance with CHBDC-06. On the other hand, the ultimate compressive strength is given as fcu =0.64fc' and the ultimate strain in 3% [8,13].
At each section, it is ensured that the factored moment and shear are less or equal to the factored flexural and shear resistances, respectively. To ensure a ductile failure at ultimate
Structural Performance of Precast Prestressed Bridge Girders Built with Ultra High Performance Concrete
prestressing steel are kept below the ultimate limit values, respectively, while the strain in prestressing steel is well beyond the yield strain.
The ultimate shear strength Vu consists of three major components: (i) the concrete
contribution, Vc; (ii) the shear reinforcement contribution, Vs; and (iii) the prestressing
reinforcement contribution through the effective prestressing force component in the direction of applied shear, Vp. For UHPC, the concrete contribution is calculated using AFGC-IR-02 Cl
7.3,21, which consists of two components: (a) the concrete contribution, VRc =0.16 fcjb0z, where b0is the web width and z is the effective depth, and (b) the fiber contribution Vf, which
is given in [8]. The shear strength of the UHPC girder is found to be sufficient to resist the applied shear force, however, minimum shear reinforcement should be provided in the critical shear zones to increase the safety against shear failure.
G1 G2 G3 G4 G5
1.225 m 2.5 m 2.5 m 2.5 m 2.5 m 1.225 m
(a) (b)
G1 G2 G3 G4
1.225 m 3.333 m 3.333 m 3.333 m 1.225 m
Figure 2: Slab-on Precast/Prestressed Girders Bridge: (a) HPC, (b) UHPC
3 Refined analysis using finite element method 3.1 Three dimensional finite element modeling
A linear elastic three-dimensional (3-D) finite element model (FEM) is used to determine the stress distribution in all girders that make up the two investigated bridges. This 3-D FEM model enabled a more accurate prediction of the stresses in all girders than the simplified analysis approach of the code used in the initial step. Both the deck slab and girders are modeled using shell elements, while the prestressing tendons are modeled using cable elements. The prestressing losses, deformations and relaxation are accounted in the model. 3.2 Results and discussions
The FEM results indicate that the maximum stresses are found in the central girders for both HPC and UHPC for the case of two lanes loading. On the other hand, the maximum stresses are found in the external girders for the case of three-lanes loading. In general, the results show that the maximum stresses for the three-lane loading case are less critical than those of the two lane loading case.
The FEM model enables predicting the stresses in every girder of the bridge and then optimize the prestressing steel ratio, , which represents the ratio of the prestressing steel area to the concrete area of the girder. Figure 3 shows the variations of the stresses at the top fiber of different girders centerline cross-section of HPC and UHPC bridges with at SLS. Five CPCI-1600 girders are used for the HPC bridge, while only four CPCI-1200 are used for the UHPC. Figure 3 shows that the most critical girders are the central girder (G3)
ps
R
ps
R
concrete. Figure 4 shows that the compressive stresses at ULS in the top fibers of the critical girders identified above are well below the ultimate stress level for both HPC and UHPC girders, while the strain in prestressing steel is well beyond the yield strain. The static deflection at SLS is found to be below (L/400) for CPCI-900 and below (L/500). Figures 3 and 4 demonstrate that the SLS requirements are controlling the design.
For all loading cases and all girders, the maximum stress zones had been identified, which are similar to HPC (for the four conventional zones at the support and mid-span sections) in addition to a zone that appears near the bottom transition area from the girder web and bottom flange near the support, which is not captured by the simplified analysis approach. In most of the cases, the concrete in this zone is subjected to compressive stresses that are higher than those at the support. Experimental tests [11] have identified a crushing failure mode in the same zone. The stresses at the top of the support zone of the HPC bridge fluctuate between compression and tension following a slight change in the prestressing steel ratio. For the UHPC bridge, the compressive stresses in all maximum stress zones vary quasi-linearly with the prestressing steel ratio. The maximum tensile stresses at the bottom of the midspan zone exhibit linear relationships with . The preliminary bridge design and stress analysis show that the maximum feasible eccentricity of the conventional deflected tendons at the support section is needed for the UHPC girders.
ps
R
A comparison of the results for CPCI 1200 and CPCI 900 shows that the stresses in the CPCI 1200 girder are relatively low and this section represents a conservative choice. On the other hand, all compressive stresses in the CPCI 900 girder are below at SLS and below at ULS, while the tensile stress at the bottom fiber of the mid-span is at its allowable limit and thus controls the design. Consequently, the prestressing area ratio needed to satisfy the non-cracking requirement is relatively high. A comparison between the two sections indicates that a more efficient section could be developed that falls between CPCI-900 and CPCI 1200.
' c f 45 . 0 ' 64 . 0 fc
4 Comparison of materials consumption in UHPC and HPC Bridges
As mentioned earlier, the use of UHPC enables a considerable reduction in the concrete volume of up to 49% for the CPCI 1200 and 65% for CPCI 900.The weight of the girders per unit area of the bridge deck are 0.481 tons/m2 for HPC bridge, 0.196 tons/m2 for UHPC – CPCI 900 girders, and 0.288 tons/m2 for UHPC –CPCI 1200 girders. The total weight per unit area of the superstructure, including the deck slab are 0.901 tons/m2 for HPC, 0.616 tons/m2 for UHPC–CPCI 900 girders. Consequently, UHPC results in 32% reduction in the total weight of the superstructure and 59.3% reduction in the girders weight. The prestressing steel area required for CPCI 900 section is 39% higher than that for CPCI 1600, which is only
Structural Performance of Precast Prestressed Bridge Girders Built with Ultra High Performance Concrete
7
superstructure will lead to a reduced size of the substructure (piers and abutments) and foundations and reduced overall cost of the bridge. Furthermore, a reduction in the concrete consumption will have considerable environmental benefits through the reduction of energy consumption and greenhouse gas emission (GHG) associated with the production of cement, extraction and transportation to the construction site of raw materials [12].
- 0 . 0 6 - 0 . 0 4 - 0 . 0 2 0 0 . 0 2 0 . 0 4 0 . 0 6 0 . 0 8 0 . 1 0 . 1 2 0 . 1 4 0 . 1 6 0 . 1 8 0 . 2 9 1 2 1 5 1 8 2 1 2 4 2 7 3 Rp s X 1 0 0 0
Normalized maximum stresses 0
H P C - G 1 H P C -G 2 H P C - G 3 H P C -G 4 H P C - G 5 U H P C - C P C I- 1 2 0 0 - G 1 U H P C - C P C I-1 2 0 0 - G 2 U H P C - C P C I- 1 2 0 0 - G 3 U H P C - C P C I-1 2 0 0 - G 4 ) ( ' HPC f f c t O p t im u m P r e s tr e s s in g S t e e l A r e a '(UHPC) f f c t
Figure 3: Variation of maximum SLS stresses with prestressing steel ratio for all girders
- 0 . 4 5 - 0 . 4 - 0 . 3 5 - 0 . 3 - 0 . 2 5 - 0 . 2 - 0 . 1 5 - 0 . 1 - 0 . 0 5 0 0 . 0 5 9 1 2 1 5 1 8 2 1 2 4 2 7 Rp s X 1 0 0 0 Normaliz ed Maximum Stresses 3 0 H P C - G 1 H P C - G 2 H P C - G 3 H P C - G 4 H P C - G 5 U H P C - C P C I - 1 2 0 0 - T & B - G 1 U H P C - C P C I - 1 2 0 0 - T & B - G 2 U H P C - C P C I - 1 2 0 0 - T & B - G 3 U H P C - C P C I - 1 2 0 0 - T & B - G 4
Figure 4: Variation of maximum ULS stresses with prestressing steel ratio for all girders
5 Conclusions
The use of UHPC in precast/prestressed concrete girders enables significant reductions in the number of girders and girder size when compared to conventional HPC bridge girders. It can also enable longer spans without further increase in the girder size. This study shows that UHPC yields a considerable reduction in the concrete volume from 48.7% to 65%. The finite element model shows that the stress distributions in the UHPC bridge girders yield
patterns similar to those of the HPC bridge girders. The stress distribution in the girder and the regions of stress concentration do not agree perfectly with the results obtained using the simplified load distribution method of CHBDC-06. Further investigations are needed to develop a simplified approach capable to accurately capture the extreme stresses in UHPC bridges.
A comparison between the two UHPC examined sections shows that an optimum section can be developed that is between CPCI-900 and CPCI 1200 can be achieved by increasing the section modulus. This would improve the girder capacity without adding higher concrete weight.
6 References
[1] Lounis, Z., and Cohn, M.Z., “Optimization of Precast Prestressed Bridge Girder Systems”, PCI Journal, V. 38, No. 4, 1993, pp 60-77.
[2] Lounis, Z., and Mirza, M.S., “High Strength Concrete in Spliced Prestressed Concrete Bridge girders.” Proc. of PCI/FHWA Int. Symp. on High Performance Concrete, 1997, pp.39-59.
[3] Acker, P., and Behloul, M., “ Ductal® Technology: A Large Spectrum of Properties, A Wide Range of Application”, Proc. of the Int. Symp. on UHPC Kassel, Germany, 2004, pp.11-23.
[4] Buitelaar, P., “Heavy Reinforced Ultra High Performance Concrete”, Proceedings of the Int. Symp. on UHPC, Kassel, Germany, September 13-15, 2004, pp.25-35.
[5] Hajar, Z., Lecointre, D., Simon, A., and Petitjean, J. “Design and Construction of the World First Ultra-High Performance Concrete Road Bridges”, Proceedings of the Int. Symp. on UHPC, Kassel, Germany, September 13-15, 2004, pp.39-48.
[6] Meda, A., Rosati, G., “Design and Construction of a Bridge in Very High Performance Fiber Reinforced Concrete”, Journal of Bridge Engineering, Vol. 8, No. 5, 2003, pp.281-287.
[7] Bierwagen, D., and Abu-Hawash, A., “Ultra High Performance Concrete Highway Bridge”, Proc. of the 2005 Mid-Continent Transportation Research Symposium, Ames, Iowa, 2005, pp.1-14. [8] AFGC Groupe de travail BFFUP, “Ultra High Performance Fiber-Reinforced Concretes: Interim
Recommendations”, Scientific and Technical Committee, Association Française de Genie Civil, 2002.
[9] Canadian Prestressed Concrete Institute, “Design Manual, Precast and Prestressed Concrete”, Third Edition, 1996.
[10] Canadian Standards Association, CAN/CSA-S6-06, “ Canadian Highway Bridge Design Code”, 2006.
[11] U.S. Department of Transportation, Federal Highway Administration, “ Structural Behavior of UHPC Prestressed I-Girders”, Publication No. FHWA-HRT-06-115, 2006.
[12] Daigle, L., and Lounis, Z., “Life Cycle Cost Analysis of HPC Bridges Considering their Environmental Impact”, Proc. of INFRA 2006, Quebec City, pp. 1-17.
[13] Almansour,H, Lounis, Z., “ Innovative Precast Bridge Superstructure Using Ultra High Performance Concrete Girders“, Proc. of PCI 53rd National Bridge Conference, 2007.
![Table 1 Properties of Investigated Standard CPCI I-Sections [9]](https://thumb-eu.123doks.com/thumbv2/123doknet/14165554.473817/6.892.102.805.471.627/table-properties-investigated-standard-cpci-i-sections.webp)
